High refractive index ophthalmic eyeglass lenses are attractive options for their increased refractive power and decreased thickness. However, the common use of low refractive index backside hard-coatings creates interference fringes on such lenses which can strongly detract from the appearance of the lens.
Common ophthalmic lenses are made from relatively soft polymeric materials prone to scratching. Hence, in order to provide adequate surface robustness, lenses must be coated with a scratch resistant coating. Such “hard-coatings” are often formed from urethane lacquers, siloxane polymers, or colloidal dispersions of oxide nano-particles, such as silicon dioxide, SiO2. These layers are usually deposited by dip based processes, coating both sides of the lens. A typical thickness of the coating is in the range of 1-5 micrometers in order to achieve the desired abrasion resistance with high adhesion and resistance to cracking. In this range of thickness, the hard-coatings can have an effect on the cosmetic performance of the lens. This is especially the case when the hard-coating is not closely index matched to the lens material. The reflection from an interface is given by Formula A:
  R  =            [                        (                                    n              1                        -                          n              o                                )                          (                                    n              1                        +                          n              o                                )                    ]        2  
Where no is the index of the incident medium (typically air) and n1 is the index of the other surface forming the interface. Consider the example of a high index lens (n1=1.70) and an incident medium of air (no=1). The reflection at this interface can then be calculated as 6.72%. This is for a single interface; hence, a total reflection of the lens is 13.4%. This shows the importance of antireflection coatings to lower this number and increase the transmission. The most common hard-coatings have an index of 1.5, significantly different from the 1.70 lens. The reflection from the hard-coating is lower than the reflection from the lens surface. While this would initially seem beneficial (lowering the reflection), the interface between the hard-coating and the lens also generates a reflection which must also be considered. The different reflections are shown below in Table 1.
TABLE 1Interfacen1noReflectance (%)Lens - Air1.701.006.72Hard-Coating - Air1.501.004.00Lens - Hard-Coating1.701.500.39
While the magnitude of the reflection from an interface between a 1.7 (lens) and 1.5 (hard-coating) index step is much smaller than the other reflections it has a significant impact on the reflected spectrum from the lens. Since light is a wave the reflections from the different interfaces combine destructively or constructively depending on the wavelength of light. For example, in FIG. 1, the straight line is the reflection from a single surface of a 1.67 index lens with an index matched hard-coating. The oscillating line is the single surface reflection of a 1.67 index lens with a 1.5 index, mismatched hard-coating that is 4 micrometers thick. As can be seen, the effect is very large—varying the reflection from over 6% to almost as low as 2%. These oscillations give rise to a coloration of the lens surface. Since typical hard-coatings are deposited by dip processes, they are not perfectly uniform which means the reflection spectra changes over the lens surface. This, in turn, generates interference effects similar to the fringes observed on a soap bubble and significantly detracts from the appearance and performance of a lens
The above example demonstrates the advantages to using index matched hard-coatings for any given lens material. However, there are several limitations that can prevent this from occurring. First, hard-coatings are not available in all commercially used lens material indices. There is a trend towards increasing lens indices to better serve individuals requiring high refractive powers and to allow thinner more attractive lenses. However, new lens material development is often in advance or ahead of the development of hard-coatings having similar indices. Therefore, it is often the case that lens materials new to the market place do not have closely refractive index matched hard-coatings available.
The second limitation relates to processing a lens blank into a finished lens. When a semi-finished lens is made into a specific prescription for a patient, the back surface of the lens is ground away and polished to a specific curve to generate the desired optical refracting power. The removal of material from the back surface of the lens also removes the factory applied hard-coating. In most labs, a new or replacement backside hard-coating is applied via spin coating and UV curing. The available UV cured, spin coat, hard-coatings are mostly based on siloxane chemistries which limits the refractive index to around 1.5. Accordingly, in the case of high index lenses, the effect described above occurs on the back surface of the lens. Because high index materials have increased refractive power relative in comparison to common lens materials, such as CR39, high index lens materials are often employed for prescriptions with high corrective power. However, the increased refractive power also allows the lens thickness to be decreased which many eyeglass wearers find attractive. Unfortunately, this desirable decrease in lens thickness is often accompanied by the degradation in the appearance of the lens due to the index mismatch between the hard-coating and the lens.
Applying an antireflective, AR, coating to such a lens can make the above-described problem worse. The oscillation in the reflectance is still present in the reflection spectrum, albeit with a reduced amplitude. However, the amplitude of the oscillation is similar in magnitude to the reflection from the AR coating. This can result in distortion of the color of the antireflection coating. Since the hard-coating thickness changes over the lens surface, the distortion changes over the lens surface making it very noticeable. This effect is shown in FIGS. 2A and 2B which provide a comparison of a reflection spectrum of a lens having an antireflection coating with an index matched hard-coating (FIG. 2A) versus a lens having an antireflection coating with a mismatched hard-coating, i.e. a 1.5 index hard-coating on a 1.67 index lens (FIG. 2B). The straight lines in FIGS. 2A and 2B represent the reflection from the surface of the lens without a hard coating or an anti reflective coating applied.
A third limitation relates to the use of index matching hard-coatings for photochromic lenses. Photochromic lenses contain one or more photochromic dyes in a layer on top of a lens, in a laminate embedded inside of a lens, or dispersed within the bulk lens material. When exposed to a specific wavelength band of light, the dyes undergo a reversible transformation between a clear, high light-transmitting state and a darkened, reduced light-transmitting state. In ophthalmic lenses, this functionality is employed to make lenses darken when used outside in sunlight and clear or clearer when used indoors. The wavelengths of light responsible for this color transformation are in the UV spectrum, typically between 300 to 400 nm. This creates a potential problem when the photochromic functionality is combined with high index hard-coatings.
For example, to achieve high refractive indices, commercially available hard-coatings contain dopants such as TiO2 or thio-urethanes. Both materials are UV absorbing and reduce the amount of UV light reaching a photochromic dye employed on or within the photochromic lens. This is shown in FIG. 3 which compares the transmission spectrum of two commercially available hard-coatings; HC-A having a refractive index near 1.50 (low index) and HC-B having a refractive index near 1.67 (high index). The reduced transmission of HC-B from 300 to 380 nm will degrade the performance of a UV activated photochromic material. To maintain the performance, it would therefore be necessary to use a coating more like HC-A. However, this would then lead to the same color variation and degraded appearance as described above.
In view of the above, it becomes apparent that there is a need in the field for a means of utilizing the available and commercially common low index hard-coatings with high index lenses and other optical substrates while minimizing the undesirable optical effects, e.g. interference fringes, of the typical mismatched refractive indices of the hard-coating and high index optical substrates.